Conventionally known coordinate input apparatuses sense a light spot on a screen using a CCD area sensor or linear sensor, process the image using barycentric coordinates or pattern matching to calculate the coordinate values, and output them, or use a position detection element (an analog device capable of obtaining an output voltage corresponding to the position of a spot) called a PSD.
For example, Japanese Patent Publication No. 7-76902 discloses an apparatus which senses a light spot formed by a visible parallel beam with a video camera to detect coordinates and simultaneously transmits/receives control signals using infrared diffused light. Japanese Patent Laid-Open No. 6-274266 discloses an apparatus for detecting coordinates using a linear CCD sensor and special optical mask.
Japanese Patent No. 2503182 discloses the arrangement of an apparatus using a PSD and an output coordinate correction method.
In recent years, there is a great demand for large screen displays because they have improved brightness and therefore can be sufficiently used in an illuminated environment. Coordinate input apparatuses are also increasingly required to avoid the influence of disturbance light for use in combination with such large screen displays. In recent years, the number of devices as radio communication means using infrared light is increasing. Since disturbance light tends to increase in both infrared light and visible light, an important characteristic of an apparatus is that the apparatus is not affected by disturbance light.
However, a coordinate input apparatus using a CCD sensor, as disclosed in Japanese Patent Publication No. 7-76902 or Japanese Patent Laid-Open No. 6-274266, can suppress disturbance light only with an optical filter.
To the contrary, a coordinate input apparatus using a PSD, as disclosed in Japanese Patent No. 2503182, can stand disturbance light in cooperation with an optical filter because the influence of disturbance light can be suppressed by frequency-modulating the light intensity and synchronously detecting the modulated wave.
For a large screen display, not only the brightness but also resolution is being improved simultaneously. For this reason, the resolving power of a coordinate input apparatus also need be improved. The coordinate input apparatus using a PSD, which can stand disturbance light, has a problem on this point. More specifically, the dynamic range of a sensor output voltage directly corresponds to the input range. For this reason, for example, to decompose an entire image into 1,000 coordinates, an S/N ratio of at least 60 dB is necessary. In addition, as disclosed in Japanese Patent No. 2503182, a linear error must be digitally corrected, so a highly accurate analog circuit and a multi-bit A/D converter and calculation circuit are required. Furthermore, since the S/N ratio of a sensor output signal depends on the light amount and sharpness of the light spot, only the above-described disturbance light suppression is insufficient, and a bright and highly accurate optical system is also required. Hence, the apparatus itself becomes very expensive and bulky.
As a method of increasing the resolving power using a CCD sensor, Japanese Patent Publication No. 7-76902 discloses a method of using a plurality of video cameras. However, this makes the apparatus bulky and expensive. When one video camera with a large number of pixels is used, the apparatus becomes much more bulky and expensive than that using a plurality of cameras. To achieve resolving power more than the number of pixels by image processing, an enormous quantity of image data must be processed at a high speed. For this reason, an apparatus for real-time operation becomes very bulky and expensive.
In Japanese Patent Laid-Open No. 6-274266, high resolving power can be obtained using a special optical mask and signal processing. The resolving power can be made high if disturbance light is small, and a satisfactory S/N ratio can be ensured. In fact, however, since a linear sensor forms a linear image and cannot separate it from disturbance light in plane, unlike an area sensor for forming a point image, the apparatus is readily affected by disturbance light and can be put into practical use in only a special environment with small disturbance light.
In combination of a coordinate input apparatus and projection-type large screen display, as a known technique of ensuring a wide viewing angle on the front surface as an input side and also a screen observation side of the screen used for the projection-type large screen display, a screen having a Fresnel lens and lenticular lens surface is used, and a diffusion material is used together with the screen. Additionally, as disclosed in Japanese Patent Laid-Open No. 58-59436, a screen arrangement for improving contrast is known in which black stripes 13-2 opposing concave portions on the rear surface of a lenticular lens 14-1 are formed on the observation side (image light output side) of the lenticular lens, as shown in FIG. 22. However, in detecting the coordinates, on the screen, of a light spot from a pointing tool such as a pen, the light input to the sensor is shielded by the black stripes, and no sufficient detection light amount can be ensured.